1. Field of the Invention
This invention relates to a method for estimating hold time sweep crack growth properties of IN718 cast HIPed material.
2. Description of the Prior Art
Inconel Alloy 718 (IN718) has the major chemistry of Nixe2x80x94Fexe2x80x94Crxe2x80x94Cbxe2x80x94Moxe2x80x94Tixe2x80x94Al and was developed through extensive optimization studies by H. L. Eiselstein at the International Nickel Company (INCO) in the 1950""s. Alloy IN718 is a precipitation hardenable nickel based alloy with high strength and ductility at temperatures up to 704xc2x0 C., good corrosion resistance, ease of formability and can be welded with good resistance to strain-age cracking. Alloy IN718 was initially developed for the aerospace industry, and it has been used for jet engine and high-speed airframe parts such as wheels, buckets, spacers, and high temperature bolts and fasteners. IN718 investment cast HIPed material is a new approach in making manifold for steam delivery system in GE H technology gas turbines.
Investment casting, often called lost wax casting, is regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy. The most common use of investment casting in more recent history has been the production of components requiring complex, often thin-wall castings. The investment casting process normally includes the following steps: creating a wax pattern, assembling the wax pattern cluster, xe2x80x9cinvestingxe2x80x9d the cluster with ceramic stucco/slurry; de-waxing and fire molding the ceramic for strength, melting the alloy in vacuum or air; pouring molten alloy into the mold; knocking off the shell and heat treating/machining/coating operations. An HIP process (Hot Isostatic Pressing) sometimes follows the investment casting process to consolidate shrinkage porosity internal to casting and help homogenize structure.
GE Power Systems introduced H technology gas turbines in 1995. H technology is a platform of combined-cycle technology that integrates the gas turbine, steam turbine and generator into a seamless system, where each component is optimized for the highest level of performance. The centerpiece of this new technology platform is an advanced closed-loop steam cooling system in the gas turbine which permits higher firing temperature while retaining combustion temperatures at levels consistent with low emissions. That will enable the new machines to operate at firing temperatures in the 2,600F. class, leading to 60% net thermal efficiency and world record output for a combined-cycle unit. Unlike aircraft engines, which only have air for cooling, a combined-cycle system has ready steam supply. That steam is captured and used for cooling in this closed-loop system. Steam is desired because it has a higher heat capacity than air. The steam cooling system uses a manifold as a critical component. However, it is difficult to make the manifold due to its complex geometry. See FIG. 1.
Originally, manifolds were made by machining forged alloy IN718 material to the desired shape. This process can be expensive and take a long time. Other processes were investigated to reduce the cost and cycle time including the HIPed (Hot Isostatic Press) investment casting process. Manifolds produced by the investment cast process of alloy IN718 have lower costs, larger yields and reduced cycle time.
However, in designing a manifold, hold time sweep crack growth rate must be considered. It is fatal to have fast crack growth rate for manifold with thin walls. Because the crack growth rate was unknown for superalloy IN718 material formed by the HIPed investment cast process, it has been difficult to design manifolds with this material. Moreover, the manifold is working in steam environment and the effect of steam on hold time crack growth rate was also unknown.
Crack growth rate can be affected by many factors such as process parameters, grain size, hold time, and temperature. Materials made by investment casting can have varied grain sizes. For example, thin sections or areas close to the mold can have fine grains, while the center of thick sections can have coarse grain size. Forging, on the other hand, typically has uniform grain size all over the part. In addition, duration of hold time, and the type of environment (steam or air) can also affect the crack growth rate.
The relationship between HIPed investment cast IN718 hold time crack growth rate properties and the corresponding grain size, environment, and hold time length is desirable for the design and life evaluation of a manifold. However, hold time sweep crack growth rate data was heretofore not available for cast HIPed 718 material, and there was no known method to estimate the crack growth rate. Moreover the effect of steam on the cast HIPed 718 material with various grain sizes was unknown.
The invention relates to a method to estimate the crack growth rate of HIPed cast IN718 material with various grain sizes and working environments. In particular, the invention is directed to a method of estimating the crack growth rate of an HIPed IN718 investment cast component in an air or steam environment comprising determining the average grain size diameter and duration of hold time and solving the following equation,             ⅆ      a              ⅆ      N        =      1                                                      1.19              xc3x97                              10                5                            xc3x97              GS                        +                          5.59              xc3x97                              10                4                            xc3x97              Env                        -                                                                          1.17              xc3x97                              10                5                            xc3x97              GS              xc3x97              Env                        +                          0.66              xc3x97              HT                        -                          7.267              xc3x97              GS              xc3x97              HT                                          
whereby N is the number of operating cycles, a is the crack length, da/dN is crack growth rate, GS is the average grain size in meters, HT is hold time in seconds, Env is 1 for an air environment, and Env is xe2x88x921 for a steam environment.
The estimates obtained from the transfer function can be used for designing of and predicting the life of cast IN718 components such as manifolds.